1) Access the Customer Portal (you should have receive an invitation to the portal). If you don’t have the link, I can send you an invitation again.
2) On the main screen, Click on the “View More” link on “My Details” section:

3) On the Payment Details section, click to add/edit your credit/debit card info

4) Invoices will be charged to that card 10 days after you receive the invoice automatically.